The present invention relates in general to a thermal extensometer and pertains more particularly to a device for measuring deformations with an exceptionally high degree of resolution. The device of this invention is characterized by an inherent insensitivity of the device to temperature variations with the insensitivity being of the same order as the degree of resolution.
There are many types of devices available for measuring deformation but most are limited by poor stability, poor resolution, temperature sensitivity or limited operating temperature range. On the contrary, the device of the present invention is particularly suited to the measurement of very small minute temperature-induced deformations.
There are currently in existence a number of techniques and types of apparatus for measuring and determining thermal expansion. Some of these techniques require a material test sample of a special size and shape and employ a special heating oven equipped with a means of transmitting the thermal growth to a deflection sensing device operating at room temperature. The requirement for a special size and shape sample is inconvenient and sometimes difficult. The means of transmitting the thermal growth to a deflection sensing device operating at room temperature can introduce sensitivity, hysteresis and mechanical loading errors.
Another technique employs interferometric principles. This type of a measuring technique can measure very small deflections such as those occurring during material phase transformation. However, interferometric devices are usually expensive, complicated, fragile, not very portable and require inplane line-of-sight access between the heated sample under investigation and the deflection sensing optics or electronics. Also, heated air and optical windows used in conjunction with interferometric techniques cause diffraction and refraction of the light, resulting in troublesome errors.
In U.S. application Ser. No. 296,969, now abandoned there is shown a strain-measuring device that measures stress-induced strain while inherently cancelling thermal expansion strain. The thermal expansion strain cancellation is accomplished by fabricating the link from a material that has a thermal expansion characteristic that matches that of the specimen. On the other hand, in accordance with the principles of this invention the desire is not to measure stress-induced strain, but rather to measure expansion due to thermal variations.
The device of the present invention is inherently insensitive to temperature changes and is capable of operating in a vacuum and at extreme temperatures, including very high temperatures. The device of this invention is portable and attachable to the surface of a material without the requirement that the material be of any specific size or configuration. Therefore, one or more of the devices can be attached to sample coupons, fabricated parts, structures, structural components or mechanical equipment, and when attached to these parts or specimens is used to measure temperature induced deformations when the material to which it is attached changes temperature.
The use of the device of this invention generally falls into two categories. First, the device is used to measure minute deformations in deformation-sensitive structures like bridges, buildings and even spacecraft when they are subjected to cyclic or asymmetric heating. For example, the deformations taking place in a beam of a building or in a bridge span over a 24-hour period or even a one-year period can be continuously measured by the device and used to evaluate the design and operation of expansion joints.
Another category of practical use of the device of this invention is as a dilatometer for measuring the thermal expansion strain and determining the thermal coefficient of expansion of unstressed materials. Thermal expansion, or the thermal co-efficient of expansion is a basic physical property of materials. It is an important consideration in the design and function of mechanical equipment, optical devices, structures and other types of hardware. The applicability and high resolution of the device of this invention provides the opportunity to mount several units on a material sample and measure anisotropic thermal expansions simultaneously.